/*******************************************************************************
 * Copyright 2011 See AUTHORS file.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 ******************************************************************************/

package com.badlogic.gdx.utils;

import java.util.Arrays;
import java.util.Iterator;
import java.util.NoSuchElementException;

import com.badlogic.gdx.math.MathUtils;

/** An unordered set where the keys are objects. Null keys are not allowed. No allocation is done except when growing the table
 * size.
 * <p>
 * This class performs fast contains and remove (typically O(1), worst case O(n) but that is rare in practice). Add may be
 * slightly slower, depending on hash collisions. Hashcodes are rehashed to reduce collisions and the need to resize. Load factors
 * greater than 0.91 greatly increase the chances to resize to the next higher POT size.
 * <p>
 * Unordered sets and maps are not designed to provide especially fast iteration. Iteration is faster with OrderedSet and
 * OrderedMap.
 * <p>
 * This implementation uses linear probing with the backward shift algorithm for removal. Hashcodes are rehashed using Fibonacci
 * hashing, instead of the more common power-of-two mask, to better distribute poor hashCodes (see <a href=
 * "https://probablydance.com/2018/06/16/fibonacci-hashing-the-optimization-that-the-world-forgot-or-a-better-alternative-to-integer-modulo/">Malte
 * Skarupke's blog post</a>). Linear probing continues to work even when all hashCodes collide, just more slowly.
 * @author Nathan Sweet
 * @author Tommy Ettinger */
public class ObjectSet<T> implements Iterable<T> {
    public int size;

    T[] keyTable;

    float loadFactor;
    int threshold;

    /** Used by {@link #place(Object)} to bit shift the upper bits of a {@code long} into a usable range (&gt;= 0 and &lt;=
     * {@link #mask}). The shift can be negative, which is convenient to match the number of bits in mask: if mask is a 7-bit
     * number, a shift of -7 shifts the upper 7 bits into the lowest 7 positions. This class sets the shift &gt; 32 and &lt; 64,
     * which if used with an int will still move the upper bits of an int to the lower bits due to Java's implicit modulus on
     * shifts.
     * <p>
     * {@link #mask} can also be used to mask the low bits of a number, which may be faster for some hashcodes, if
     * {@link #place(Object)} is overridden. */
    protected int shift;

    /** A bitmask used to confine hashcodes to the size of the table. Must be all 1 bits in its low positions, ie a power of two
     * minus 1. If {@link #place(Object)} is overriden, this can be used instead of {@link #shift} to isolate usable bits of a
     * hash. */
    protected int mask;

    private transient ObjectSetIterator iterator1, iterator2;

    /** Creates a new set with an initial capacity of 51 and a load factor of 0.8. */
    public ObjectSet () {
        this(51, 0.8f);
    }

    /** Creates a new set with a load factor of 0.8.
     * @param initialCapacity If not a power of two, it is increased to the next nearest power of two. */
    public ObjectSet (int initialCapacity) {
        this(initialCapacity, 0.8f);
    }

    /** Creates a new set with the specified initial capacity and load factor. This set will hold initialCapacity items before
     * growing the backing table.
     * @param initialCapacity If not a power of two, it is increased to the next nearest power of two. */
    public ObjectSet (int initialCapacity, float loadFactor) {
        if (loadFactor <= 0f || loadFactor >= 1f)
            throw new IllegalArgumentException("loadFactor must be > 0 and < 1: " + loadFactor);
        this.loadFactor = loadFactor;

        int tableSize = tableSize(initialCapacity, loadFactor);
        threshold = (int)(tableSize * loadFactor);
        mask = tableSize - 1;
        shift = Long.numberOfLeadingZeros(mask);

        keyTable = (T[])new Object[tableSize];
    }

    /** Creates a new set identical to the specified set. */
    public ObjectSet (ObjectSet<? extends T> set) {
        this((int)(set.keyTable.length * set.loadFactor), set.loadFactor);
        System.arraycopy(set.keyTable, 0, keyTable, 0, set.keyTable.length);
        size = set.size;
    }

    /** Returns an index >= 0 and <= {@link #mask} for the specified {@code item}.
     * <p>
     * The default implementation uses Fibonacci hashing on the item's {@link Object#hashCode()}: the hashcode is multiplied by a
     * long constant (2 to the 64th, divided by the golden ratio) then the uppermost bits are shifted into the lowest positions to
     * obtain an index in the desired range. Multiplication by a long may be slower than int (eg on GWT) but greatly improves
     * rehashing, allowing even very poor hashcodes, such as those that only differ in their upper bits, to be used without high
     * collision rates. Fibonacci hashing has increased collision rates when all or most hashcodes are multiples of larger
     * Fibonacci numbers (see <a href=
     * "https://probablydance.com/2018/06/16/fibonacci-hashing-the-optimization-that-the-world-forgot-or-a-better-alternative-to-integer-modulo/">Malte
     * Skarupke's blog post</a>).
     * <p>
     * This method can be overriden to customizing hashing. This may be useful eg in the unlikely event that most hashcodes are
     * Fibonacci numbers, if keys provide poor or incorrect hashcodes, or to simplify hashing if keys provide high quality
     * hashcodes and don't need Fibonacci hashing: {@code return item.hashCode() & mask;} */
    protected int place (T item) {
        return (int)(item.hashCode() * 0x9E3779B97F4A7C15L >>> shift);
    }

    /** Returns the index of the key if already present, else -(index + 1) for the next empty index. This can be overridden in this
     * pacakge to compare for equality differently than {@link Object#equals(Object)}. */
    int locateKey (T key) {
        if (key == null) throw new IllegalArgumentException("key cannot be null.");
        T[] keyTable = this.keyTable;
        for (int i = place(key);; i = i + 1 & mask) {
            T other = keyTable[i];
            if (other == null) return -(i + 1); // Empty space is available.
            if (other.equals(key)) return i; // Same key was found.
        }
    }

    /** Returns true if the key was not already in the set. If this set already contains the key, the call leaves the set unchanged
     * and returns false. */
    public boolean add (T key) {
        int i = locateKey(key);
        if (i >= 0) return false; // Existing key was found.
        i = -(i + 1); // Empty space was found.
        keyTable[i] = key;
        if (++size >= threshold) resize(keyTable.length << 1);
        return true;
    }

    public void addAll (Array<? extends T> array) {
        addAll(array.items, 0, array.size);
    }

    public void addAll (Array<? extends T> array, int offset, int length) {
        if (offset + length > array.size)
            throw new IllegalArgumentException("offset + length must be <= size: " + offset + " + " + length + " <= " + array.size);
        addAll(array.items, offset, length);
    }

    public boolean addAll (T... array) {
        return addAll(array, 0, array.length);
    }

    public boolean addAll (T[] array, int offset, int length) {
        ensureCapacity(length);
        int oldSize = size;
        for (int i = offset, n = i + length; i < n; i++)
            add(array[i]);
        return oldSize != size;
    }

    public void addAll (ObjectSet<T> set) {
        ensureCapacity(set.size);
        T[] keyTable = set.keyTable;
        for (int i = 0, n = keyTable.length; i < n; i++) {
            T key = keyTable[i];
            if (key != null) add(key);
        }
    }

    /** Skips checks for existing keys, doesn't increment size. */
    private void addResize (T key) {
        T[] keyTable = this.keyTable;
        for (int i = place(key);; i = (i + 1) & mask) {
            if (keyTable[i] == null) {
                keyTable[i] = key;
                return;
            }
        }
    }

    /** Returns true if the key was removed. */
    public boolean remove (T key) {
        int i = locateKey(key);
        if (i < 0) return false;
        T[] keyTable = this.keyTable;
        int mask = this.mask, next = i + 1 & mask;
        while ((key = keyTable[next]) != null) {
            int placement = place(key);
            if ((next - placement & mask) > (i - placement & mask)) {
                keyTable[i] = key;
                i = next;
            }
            next = next + 1 & mask;
        }
        keyTable[i] = null;
        size--;
        return true;
    }

    /** Returns true if the set has one or more items. */
    public boolean notEmpty () {
        return size > 0;
    }

    /** Returns true if the set is empty. */
    public boolean isEmpty () {
        return size == 0;
    }

    /** Reduces the size of the backing arrays to be the specified capacity / loadFactor, or less. If the capacity is already less,
     * nothing is done. If the set contains more items than the specified capacity, the next highest power of two capacity is used
     * instead. */
    public void shrink (int maximumCapacity) {
        if (maximumCapacity < 0) throw new IllegalArgumentException("maximumCapacity must be >= 0: " + maximumCapacity);
        int tableSize = tableSize(maximumCapacity, loadFactor);
        if (keyTable.length > tableSize) resize(tableSize);
    }

    /** Clears the set and reduces the size of the backing arrays to be the specified capacity / loadFactor, if they are larger.
     * The reduction is done by allocating new arrays, though for large arrays this can be faster than clearing the existing
     * array. */
    public void clear (int maximumCapacity) {
        int tableSize = tableSize(maximumCapacity, loadFactor);
        if (keyTable.length <= tableSize) {
            clear();
            return;
        }
        size = 0;
        resize(tableSize);
    }

    /** Clears the set, leaving the backing arrays at the current capacity. When the capacity is high and the population is low,
     * iteration can be unnecessarily slow. {@link #clear(int)} can be used to reduce the capacity. */
    public void clear () {
        if (size == 0) return;
        size = 0;
        Arrays.fill(keyTable, null);
    }

    public boolean contains (T key) {
        return locateKey(key) >= 0;
    }

    public @Null T get (T key) {
        int i = locateKey(key);
        return i < 0 ? null : keyTable[i];
    }

    public T first () {
        T[] keyTable = this.keyTable;
        for (int i = 0, n = keyTable.length; i < n; i++)
            if (keyTable[i] != null) return keyTable[i];
        throw new IllegalStateException("ObjectSet is empty.");
    }

    /** Increases the size of the backing array to accommodate the specified number of additional items / loadFactor. Useful before
     * adding many items to avoid multiple backing array resizes. */
    public void ensureCapacity (int additionalCapacity) {
        int tableSize = tableSize(size + additionalCapacity, loadFactor);
        if (keyTable.length < tableSize) resize(tableSize);
    }

    private void resize (int newSize) {
        int oldCapacity = keyTable.length;
        threshold = (int)(newSize * loadFactor);
        mask = newSize - 1;
        shift = Long.numberOfLeadingZeros(mask);
        T[] oldKeyTable = keyTable;

        keyTable = (T[])(new Object[newSize]);

        if (size > 0) {
            for (int i = 0; i < oldCapacity; i++) {
                T key = oldKeyTable[i];
                if (key != null) addResize(key);
            }
        }
    }

    public int hashCode () {
        int h = size;
        T[] keyTable = this.keyTable;
        for (int i = 0, n = keyTable.length; i < n; i++) {
            T key = keyTable[i];
            if (key != null) h += key.hashCode();
        }
        return h;
    }

    public boolean equals (Object obj) {
        if (!(obj instanceof ObjectSet)) return false;
        ObjectSet other = (ObjectSet)obj;
        if (other.size != size) return false;
        T[] keyTable = this.keyTable;
        for (int i = 0, n = keyTable.length; i < n; i++)
            if (keyTable[i] != null && !other.contains(keyTable[i])) return false;
        return true;
    }

    public String toString () {
        return '{' + toString(", ") + '}';
    }

    public String toString (String separator) {
        if (size == 0) return "";
        java.lang.StringBuilder buffer = new java.lang.StringBuilder(32);
        T[] keyTable = this.keyTable;
        int i = keyTable.length;
        while (i-- > 0) {
            T key = keyTable[i];
            if (key == null) continue;
            buffer.append(key == this ? "(this)" : key);
            break;
        }
        while (i-- > 0) {
            T key = keyTable[i];
            if (key == null) continue;
            buffer.append(separator);
            buffer.append(key == this ? "(this)" : key);
        }
        return buffer.toString();
    }

    /** Returns an iterator for the keys in the set. Remove is supported.
     * <p>
     * If {@link Collections#allocateIterators} is false, the same iterator instance is returned each time this method is called.
     * Use the {@link ObjectSetIterator} constructor for nested or multithreaded iteration. */
    public ObjectSetIterator<T> iterator () {
        if (Collections.allocateIterators) return new ObjectSetIterator(this);
        if (iterator1 == null) {
            iterator1 = new ObjectSetIterator(this);
            iterator2 = new ObjectSetIterator(this);
        }
        if (!iterator1.valid) {
            iterator1.reset();
            iterator1.valid = true;
            iterator2.valid = false;
            return iterator1;
        }
        iterator2.reset();
        iterator2.valid = true;
        iterator1.valid = false;
        return iterator2;
    }

    static public <T> ObjectSet<T> with (T... array) {
        ObjectSet<T> set = new ObjectSet<T>();
        set.addAll(array);
        return set;
    }

    static int tableSize (int capacity, float loadFactor) {
        if (capacity < 0) throw new IllegalArgumentException("capacity must be >= 0: " + capacity);
        int tableSize = MathUtils.nextPowerOfTwo(Math.max(2, (int)Math.ceil(capacity / loadFactor)));
        if (tableSize > 1 << 30) throw new IllegalArgumentException("The required capacity is too large: " + capacity);
        return tableSize;
    }

    static public class ObjectSetIterator<K> implements Iterable<K>, Iterator<K> {
        public boolean hasNext;

        final ObjectSet<K> set;
        int nextIndex, currentIndex;
        boolean valid = true;

        public ObjectSetIterator (ObjectSet<K> set) {
            this.set = set;
            reset();
        }

        public void reset () {
            currentIndex = -1;
            nextIndex = -1;
            findNextIndex();
        }

        private void findNextIndex () {
            K[] keyTable = set.keyTable;
            for (int n = set.keyTable.length; ++nextIndex < n;) {
                if (keyTable[nextIndex] != null) {
                    hasNext = true;
                    return;
                }
            }
            hasNext = false;
        }

        public void remove () {
            int i = currentIndex;
            if (i < 0) throw new IllegalStateException("next must be called before remove.");
            K[] keyTable = set.keyTable;
            int mask = set.mask, next = i + 1 & mask;
            K key;
            while ((key = keyTable[next]) != null) {
                int placement = set.place(key);
                if ((next - placement & mask) > (i - placement & mask)) {
                    keyTable[i] = key;
                    i = next;
                }
                next = next + 1 & mask;
            }
            keyTable[i] = null;
            set.size--;
            if (i != currentIndex) --nextIndex;
            currentIndex = -1;
        }

        public boolean hasNext () {
            if (!valid) throw new GdxRuntimeException("#iterator() cannot be used nested.");
            return hasNext;
        }

        public K next () {
            if (!hasNext) throw new NoSuchElementException();
            if (!valid) throw new GdxRuntimeException("#iterator() cannot be used nested.");
            K key = set.keyTable[nextIndex];
            currentIndex = nextIndex;
            findNextIndex();
            return key;
        }

        public ObjectSetIterator<K> iterator () {
            return this;
        }

        /** Adds the remaining values to the array. */
        public Array<K> toArray (Array<K> array) {
            while (hasNext)
                array.add(next());
            return array;
        }

        /** Returns a new array containing the remaining values. */
        public Array<K> toArray () {
            return toArray(new Array<K>(true, set.size));
        }
    }
}